a1 Institut de Physique du Globe, 4 place Jussieu, 75252 Paris cedex 05, France
a2 Earth, Atmospheric and Planetary Science, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139-4307, USA
a3 Institut Jean le Rond d'Alembert, CNRS, Université Pierre et Marie Curie, Boîte 161–162, Tour 55–65, 4 Place Jussieu, 75252 Paris Cedex 05, France
The present paper is devoted to the formation of sand patterns by laminar flows. It focuses on the rhomboid beach pattern, formed during the backswash. A recent bedload transport model, based on a moving-grains balance, is generalized in three dimensions for viscous flows. The water flow is modelled by the full incompressible Navier–Stokes equations with a free surface. A linear stability analysis then shows the simultaneous existence of two distinct instabilities, namely ripples and bars. The comparison of the bar instability characteristics with laboratory rhomboid patterns indicates that the latter could result from the nonlinear evolution of unstable bars. This result, together with the sensibility of the stability analysis with respect to the parameters of the transport law, suggests that the rhomboid pattern could help improving sediment transport models, so critical to geomorphologists.
(Received December 05 2008)
(Revised August 25 2009)
(Accepted August 28 2009)